1. Field of the Invention
This invention relates to a method of forming a colored oxide film on the surface of aluminum or an aluminum alloy, and more particularly to a method of forming a colored oxide film on the surface of aluminum or an aluminum alloy by effecting electrolysis in an electrolytic bath composed of an aqueous solution containing sulfuric acid, with the aluminum or the aluminum alloy being used as an anode or both as an anode and as a cathode.
2. Description of the Prior Art
In the prior art, an aluminum material is made resistant to corrosion or the like by subjecting it to an anodic oxidation treatment in an aqueous solution containing sulfuric acid. Coloring of the oxide film formed on the aluminum material is achieved by painting it with a dye or the like, or electrolytically by impressing an AC current between the aluminum material serving as an anode and a cathode in an aqueous solution containing sulfuric acid containing a metallic salt. However, coloring with dye is defective in easy deterioration of the dye, and electrolysis with AC current involves two electrolyzing steps, and hence is disadvantageous from the economical point of view.
Further, there has also been employed a method of forming a colored oxide film by subjecting an aluminum material to an anodic oxidation treatment in an electrolytic bath composed, for example, of sulfosalicyclic acid and sulfuric acid or a metal sulfate. This method forms a colored oxide film only with one electrolytic treatment at room temperature but employs, as an electrolyte, sulfosalicylic acid which is special and expensive, so that this method is very costly. In addition, with this method, the aluminum material is used as an anode only and cannot be used as an anode and a cathode at the same time.